1. Field of the Invention
The present invention relates to an inspecting system for inspecting a printed circuit board appearance, and in particular to an improvement of an image pickup section for use in the inspection system examining a component mounting condition on a printed circuit board.
2. Description of the Prior Art
Along with recent down-sizing trend of electronic machinery apparatuses, the down-sizing with high density in mounting of electronic components has been progressed to such an extent that visual confirmation thereof in a manufacturing line process is substantially impossible, and therefore, an inspection apparatus for automatically inspecting a printed circuit board appearance in a manufacturing line process is more important. Accordingly, in the inspection apparatus for inspection of a printed circuit board appearance, it is an important requirement to realize both a low price and high speed performance in together in the apparatus enabling the automatic inspection in a manufacturing line process.
Conventionally, there was employed a two-dimensional area sensor type CCD camera capable of picking up about 512.times.480 pixels in one area pickup operation as an image pickup means in which images of an inspection object are picked up by repeating the two-dimensional movement of the CCD camera with stopping of the movement using an X-Y turn table.
In order to realize a high speed performance in inspection of an object, as compared with the conventional two-dimensional area sensor type CCD camera, a demand for employing a one-dimensional scanner linear sensor type CCD camera (hereinafter, referred to as a "line CCD camera") is increased for use in the inspection apparatus, where the images of an object are picked up while the line CCD camera is linearly moved in one-dimension, thus eliminating the stopping time of the camera in movement to thereby realize a high speed inspection.
When a line CCD camera of a line sensor type is employed in the inspection apparatus for inspection of a printed circuit board appearance, because the line CCD camera has an image pickup width of several thousands pixels in one sensing line, the images in the lateral width of a printed circuit board can be collectively picked up by a one-line sensing operation of the line CCD camera. Therefore, the movement axes of the turn table for moving the CCD camera can be reduced by one axis as compared with the conventional inspection apparatus employing a two-dimensional area sensor type CCD camera, resulting in cost reduction of the apparatus.
However, in the case of employing a line CCD camera as an image pickup means in an inspection apparatus, because a positional control with high accuracy as well as a timing control with high accuracy in image pickup operation is essentially important, it is therefore required to drive the line CCD camera or the printed circuit board of the inspection object with use of a precision mechanism table for linear movement driven by a servo-motor, and the like, which causes an increase in cost of the apparatus.
A conventional inspection apparatus for inspection of a printed circuit board appearance using a line CCD camera of a line sensor type as an image pickup means is described below with reference to FIG. 7 and FIGS. 8A-8B.
Referring to FIG. 7, the inspection apparatus includes a servo-motor 12 for linearly moving a precision mechanism table 13 (hereinafter, referred to as a "table") in one dimension. Placed on the table 13, is a printed circuit board 1 having repeat patterns of chip elements and the like which the mounting condition thereof is an object to be inspected, and the placed printed circuit board is subjected to light scanning by means of a line CCD camera 3 to pick up images thereof. A camera control section 71 includes a camera controller unit 16 for generating an image pickup start timing signal S1 and further includes a timer 17 for measuring a time of generation of the image pickup start timing signal.
The inspection apparatus further includes an image recognizing section 72 which is comprised of an image data memory 8, an image recognizing unit 9 and a determination control unit 10. Reference numeral 14 denotes a driver unit for driving the servo-motor 12. The camera control section 71, image recognizing section 72 and the driver unit 14 are operatively connected to a machine controller 73 for controlling a CRT display, key board and the like.
With respect to the above mentioned conventional inspection apparatus for inspection of a printed circuit board appearance employing a line CCD camera, the interaction between the components thereof and the operation of each component are described below.
In the first step, a printed circuit board 1 which is an object to be inspected is placed on the table 13 which is moved by the servo-motor 12 driven by the driver unit 14.
In the next step, the table 13 is moved within a range under the image pickup sensing operation of the line CCD camera 3 at a constant speed in one direction (for example, in the direction shown by an arrow A), wherein an image pickup start timing signal S.sub.1 is fed from the camera controller unit 16 to the line CCD camera 3 at predetermined time intervals measured by the timer 17 in the camera control section 71. At each time in response to the feeding of the signal S1, the image recognizing section 72 receives an image signal S.sub.2 of one line of pixels, i.e., one pixel.times.N pixels, from the line CCD camera 3 to be stored in the image memory 8, where N denotes the number of the one-line pixels picked up by the line CCD camera.
Then, the output of the image memory 8 is fed to the image recognizing unit 9 in which the supplied image signal S2 is subjected to recognition process to obtain high and low levels of the image picture signals in every cycle of the repeat patterns. Subsequently, in the final step, it is determined by the determination control unit 10 whether or not the printed circuit board appearance is good, namely, whether or not the mounting condition of elements on the printed circuit board is accurate. When a defect or difference in position exists in the printed circuit board appearance, there is generated an abnormal signal which is compared with a reference signal to indicate the existence of a defect or deviation.
By this arrangement, the line CCD camera 3 picks up the picture image of the printed circuit board 1 collectively every one line scanning with the lateral width thereof corresponding to the image pickup width of several thousand pixels of the line CCD camera. Therefore, the movement axes of the printed circuit board 1 with respect to the line CCD camera 3 can be reduced by one axis as compared to the old conventional inspection apparatus employing the two-dimensional area sensor type CCD camera, resulting in the cost reduction of the apparatus.
However, in the line sensor type inspection apparatus, the printed circuit board 1 is placed on the precision mechanism table 13 which is driven by the high precision servo-motor 12, which causes an increase in the cost. This is because, when the image pictures of the printed circuit board are picked up using the line CCD camera 3, the accuracy in position and timing of the image pickup starting operation, i.e., the accuracy in the time interval of the image pickup start timing signal S.sub.1 greatly affect the quality of the resultant image picture to be obtained.
The influence of the accuracy in position and time interval of the image pickup start timing signal on the quality of the resultant image picture is described below with reference to FIGS. 8A and 8B.
FIG. 8A shows an example of an image picture in the case where the image pickup start position with respect to the image pickup start timing signal is deviated from the accurate position. The deviation in the actual pickup position of an object al results in appearance as an elongation a2-1 and a contraction a2-2 on the resultant image picture a2, which deteriorates the image quality.
FIG. 8B shows an example in the case where the time intervals of generating the image pickup start timing signal are varied. Since the light storing time for an image picture on the CCD elements of the camera is determined by the time intervals between generation of subsequent two pulses of the image pickup start timing signal, therefore when the movement speed is high and the light storing time of sensing an object b1 is short relative to the accurate interval, there appears a dark zone b2-1 on the resultant image picture b2, and on the contrary, when the light storing time is long, there appears a bright zone b2-2 on the image picture b2. Thus, the difference in accuracy of time interval results in appearance of such as dark or bright zones of different accuracy on the resultant image picture.
These disturbances on the image picture greatly affects the inspection performance of the apparatus.